Redshift as evidence of expansion

  1. why can we believe that redshift PROOVES expansion of universe when we know that redshift could have been caused due to gravitation.
  2. jcsd
  3. Matterwave

    Matterwave 3,857
    Science Advisor
    Gold Member

    Are you suggesting that things farther away are progressively more massive so as to create a larger redshift in exactly the pattern that we would expect if these objects were simply moving away from us due to a homogeneous expansion of space?
  4. well. I guess i wanted to ask that.

  5. I dont think belief comes into it. Its the best explanation for what we can observe.
  6. The more realistic gravitational redshift alternative would be to consider a spherical wavefront expanding outward from a source. As the volume of the sphere increases it encompasses an ever increasing quantity of mass. Calculating an expected redshift at the sphere's surface for increasing radii will yield a correlated increasing redshift.

    The currently accepted model relies on a mechanism, "expanding space", which has no empirical basis. Gravitational redshifting, on the other hand has been empirically verified.
  7. Interesting. I've not heard of this. Then again, I read an article yesterday that suggested our local area of space (cluster scale) is shifting about in a flow of space causing apparent redshift, while in reality the universe isn't moving. Or at least accelerating.
  8. Chalnoth

    Chalnoth 5,622
    Science Advisor

    That's not really possible. First, you can't get redshifts close to the redshift 5-10 we see for some of the most distant objects without being right outside the event horizon of a black hole. It takes extreme space-time curvature for that to be due to a gravitational redshift at the source. Secondly, we can actually measure the masses of intermediate-distance objects via gravitational lensing, and their masses are nowhere near the amount required to generate noticeable gravitational redshifts.
  9. Can you provide a reference to that paper or article?
  10. Drakkith

    Staff: Mentor

    This only works if all the mass is on the inside of the sphere. If the universe is homogenous and isotropic, as our current view suggets, then there is approximately equal mass in every direction on a large scale.
  11. So Gauss's Law of gravity is nullified on a universal scale? I'm assuming in a finite, unbound universe every source of gravity is pulling on every other source from every possible direction owing to the wrap around, while in a infinite universe there's equal, but unwrapped gravity in all directions.

    Wouldn't this make gravitational attraction impossible? It would be like being in the center of the Earth, all the mass pulling equally each way.
  12. Chalnoth

    Chalnoth 5,622
    Science Advisor

    No, it isn't nullified. It just isn't useful in this situation. Objects within an expanding universe don't feel any attraction in any particular direction. Looking at it simply as the force on a single object, however, doesn't get you anywhere. Instead the easiest way to look at this is to consider the universe to be a uniform, compressible fluid with attractive forces between the different bits of the fluid. These internal forces will act in a similar way to pressure, causing the fluid as a whole to change how it expands (or contracts).
  13. The two things mean the same to me. Cancelled out by each other. Gauss's Law on the universe scale essentially means in every direction is an equally sized sphere of equal mass. I wasn't meaning to imply the law ceases to function.

    Having said that, how does a galaxy form if all gravitational effects are equal in all directions? Local inhomogenity? (I think that's a word). Can't the same effect redshift light?
  14. Chronos

    Chronos 10,279
    Science Advisor
    Gold Member

    The gravity of intervening masses along the route a photon follows across the universe to reach our instruments has zero effect on redshift. They bend the path [ie, gravitational lensing], but, have no effect of frequency. As photons approach a gravitational field, they are blueshifted, as they depart, they are redshifted by exactly the same amount.
  15. Chalnoth

    Chalnoth 5,622
    Science Advisor

    Right, local inhomogeneities. A region which is sufficiently overdense compared to the expansion collapses inward on itself.
  16. I do not know exactly what you mean by gravitation. But if you mean gravitational redshift, it is not compatible with Hubble linear law (unless you assume a fractal universe with D ≈ 2)
  17. Could you elaborate please, preferably without maths ;).
  18. Due to difficulties of the Big bang model to explain the observed linear relationship between redshift and distance (Hubble law), some theoreticians are seeking for alternative interpretations.

    One of these interpretations assumes that the nature of redshift is not due to space expansion but to gravitational field and obtains a linear law for an universe with a fractal distribution of matter with fractal dimension 2.

    It seems some recent surveys support a fractal distribution with D ≈ 2, but this is still open.
  19. Chalnoth

    Chalnoth 5,622
    Science Advisor

    Uh, what? In what universe is this happening?
  20. Gauss's law is not nullified, and the attraction between distant galaxies does not cancel out. That is why the density of matter in the universe causes a deceleration of the expansion.
    No it does not rely on "expanding space". Expanding space is just used to try to give an intuitive picture of what the equations of general relativity are saying. Goodness knows why, as it is a source of endless confusion.
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